Role of HNF4alpha in control of bile acid synthesis and conjugation. Among the many interesting phenotypes in the HNF4alpha liver null mice (HNF4alpha-deltaL) is an elevation in serum bile acids. HNF4alpha-deltaL mice have markedly increased levels of serum bile acids (BAs) compared with control floxed mice (HNF4alpha-F/F). Because bile acids are produced from cholesterol in liver and many enzymes involved in their biosynthesis are preferentially expressed in liver, the role of HNF4alpha in BA production was examined. This is due in part to the downregulation of genes encoding oxysterol 7alpha-hydroxylase (CYP7A1), sterol 12alpha-hydroxylase (CYP8B1), and sterol carrier protein x. CYP7A1 mRNA and protein were diminished only during the dark cycle in HNF4alphadeltaL mice, whereas expression in the light cycle was not different between HNF4alphadeltaL and HNF4alpha-F/F mice. CYP8B1 mRNA and enzyme activity was reduced regardless of light or dark cycle. An HNF4alpha binding site was found in the mouse Cyp8b1 promoter that was able to direct HNF4alpha-dependent transcription. Surprisingly, cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice. These studies reveal that HNF4alpha plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain beta-oxidation of cholesterol in vivo. HNF4alpha-deltaL mice also exhibited decreased expression of the very long chain acyl-CoA synthase-related gene (VLACSR), also called bile acid-CoA ligase, and bile acid-CoA:amino acid N-acyltransferase (BAT). This was associated with markedly elevated levels of unconjugated and glycine-conjugated bile acids in gallbladder of the HNF4alpha-deltaL mice. In agreement with this in vivo finding, HNF4? was also found to bind directly to the mouse VLACSR and BAT gene promoters, and the promoter activities were dependent on HNF4alpha-binding sites and HNF4alpha expression by direct regulation of VLACSR and BAT in vivo. These studies indicate HNF4alpha plays a central role in bile acid synthesis and conjugation and explain why the HNF4alpha-deltaL mice have abnormal bile acid homeostasis. HNF4alpha plays an important role in the maintenance of many liver-specific functions. Liver-specific HNF4alpha-null mice were used to determine whether hepatic HNF4alpha regulates blood coagulation in vivo. These mice exhibited reduced expression of hepatic coagulation factors V, IX, XI, XII, and XIIIB and a prolonged activated partial thromboplastin time but not prothrombin time. Promoter analysis of the mouse FXII and FXIIIB genes was performed to determine whether HNF4alpha directly regulates the genes encoding these coagulation factors. Sequence analysis revealed the presence of one and two HNF4alpha binding sites in the mouse FXII and FXIIIB genes, respectively. Using transient transfection and electrophoretic mobility shift analyses with the mouse FXII and FXIIIB promoters, it was established that the high levels of promoter activity were dependent on HNF4alpha binding sites and the expression of HNF4alpha. In conclusion, HNF4alpha has a critical role in blood coagulation homeostasis by directing transcription of the FXII and XIIIB genes Recent studies have uncovered transcription factor interactions in the metabolic regulation of genes by fasting. The coactivator PGC-1alpha along with HNF4alpha directly regulates expression of the constitutive androstane receptor (CAR) gene expression through a conserved HNF4-response element located in its proximal promoter. Expression of PGC-1alpha in cells increases CAR expression and ligand-independent CAR activity. Studies in HNF4alpha-null mice revealed that hepatic expression of HNF4alpha is required to produce fasting-inducible CAR expression and activity. Fasting produces increased expression of genes encoding key metabolic enzymes and an uptake transporter protein through a network of interactions involving cAMP, PGC-1alpha, HNF4alpha, CAR, and CAR target genes in liver